What does “wget -O” mean?

Here’s the man page of wget -O:

http://www.gnu.org/software/wget/manual/html_node/Download-Options.html#Download-Options

Here’s a few examples:

1. wget with no flag

   wget www.stackoverflow.com
   

   Output:

   A file named as index.html

2. wget with -O flag

   wget -O filename.html www.stackoverflow.com
   

   Output:

   A file named as filename.html

3. wget with -O- flag

   wget -O- www.stackoverflow.com
   

   Output:

   Output to stdout